Axial type fans move air, or other fluids, using rotating impeller blades. As the impeller blades rotate, different pressures on opposite sides of the blades are developed. The discharge sides of the impeller blades typically develop a high pressure while the intake sides develop a low pressure. The pressure differential between these two sides causes the fluid to flow from the high-pressure discharge side to the low-pressure intake side near the tips of the impeller blades creating an undesirable back flow of some of the fluid flow passing through the fan. It is well-known that this backflow can decrease the efficiency of the fan and may lead to undesirable noise generation.
Engine cooling fans develop static pressure across the fan such that the regions ahead of the fan are at significantly lower pressure than regions behind the fan. Many engine cooling fans have cowlings or shrouds positioned circumferentially around them in order to assist in directing the air flow in the desired direction. Practical operation of fans used in motor vehicle cooling systems dictate minimum clearances between the rotating fan members and stationary shroud members in order to ensure safe, durable functioning throughout the life of the vehicle.
Many of the cooling fan members used in such systems are ring-type fans, i.e. the fans have a circumferential ring member positioned on the tips of the fan blades. The pressures developed across the cooling fans drive leakage flow through the gaps occurring between the fan's blade tips or any rotating ring, and the stationary surfaces of the shroud.
In ring fans, the leakage flow encounters the tip gap at the trailing edge of the rotating ring and enters the gap region having a very high tangential velocity component. As the leakage flow progresses through the gap region, the viscous drag of the rotating ring continues to strengthen this vortical flow until finally it reaches the exit of the gap region, which is just upstream of the tips of the blades of the fan.
When the recirculating leakage flow reenters the main fan air flow passage, it possesses a very high tangential component, which is at odds with the velocity and direction of the primary incoming air flow of the fan. As the tangentially-oriented recirculating flow mixes with the passage of the primary air flow which is mostly axial, a vortex is formed adjacent the front of the leading edge at the tips of the fan blades. Since the leading edges of fan blades are designed for the primary flow velocity condition, the vortex encountered by the blades is misaligned relative to the intended inlet vector. This can cause the tip region to stall and the resulting low relative-momentum flow can “hang up” in the region of the blade tips and fan ring. This reduces the air flow rate of the fan, as well as its static pressure, and also increases the drag.
It would therefore be desirable to have a ring fan and shroud assembly that was effective in reducing these complications. It would further be desirable to minimize or eliminate the tangential velocity component prior to reinducing the leakage flow back into the primary air stream flowing through the fan. It would further be desirable to minimize the tip gap leakage flow and prevent tip stall.